Optical networks are used for the metro-access networks, approximately corresponding to distances lower than 100 km. Current research is in the introduction of colorless DWDM optical interfaces and transmission techniques. For example, WO2012/034604 describes self-seeding, as an example of a modulation format recently proposed for this purpose. A further example, using wavelength reuse, is described in M. Presi, R. Proietti, K. Prince, G. Contestabile, E. Ciaramella: “A Novel Line Coding Pair for Fully Passive Long Reach WDM-PONs”, ECOC 2008, 21-25 Sep. 2008, Brussels, Belgium Th.1.F.1. These solutions may have performance limitations, for example due to optical reflections or relative intensity noise.
A further system using Ultra Dense Wavelength Division Multiplexing (UDWDM) has a high cost, for example, as described in Nokia Siemens Networks: Ultra-dense WDM metro system, C2323, ITU-T Q6/15 meeting in Geneva, September 2012.
Long link distance and attenuation in metro access systems can be achieved with tunable lasers or coherent optical interfaces, for example, as described in Elton Marchena et al: Integrated Tunable CMOS Laser for Si Photonics, PDP5C.7.pdf OFC/NFOEC Postdeadline Papers© 2013 OSA. The cost issue is significant, particular for coherent interfaces, which need tunable lasers at transmitter and receiver and use a complicated receiver structure to couple the light of received signal and local oscillator.